Adaptive optical system technology has found a wide range of applications including astronomical imaging and long-range free space optical communication. Adaptive optical system technology can potentially enhance any application in which turbulence along the path, which leads to refractive index fluctuations due to temperature variations, degrades the performance of an imaging or laser projection system. Prior art methods are well known for dealing with great distances and associated phenomena of strong scintillation (wherein branch points in the phase function begin to dominate performance and amplitude fluctuations can begin to degrade performance). See references below. (1; 2; 3; 4; 5; 6; 7; 8; 9; 10). These methods suffer from two important limitations that prevent ready application for higher energy laser and directed energy applications: (1) a requirement that the phase correction device be capable of operating with a high power laser; and (2) significantly reduced effectiveness with a non-cooperative target [with the method described in reference 9 below being an exception].
On the topic of the first limitation—that the phase correction device be capable of operation with a high power laser—there are two driving issues at hand. The first driving issue is that the coating technology used to enable highly reflective coatings is at odds with the need to make the facesheet of the phase correction device very thin to enable rapid and effective correction of aberrations. The stress induced by the highly reflective coating utilized to prevent thermal induced aberrations in the correction device can lead to such strong aberrations that the phase correction device is rendered un-usable. However, if a more conventional low stress coating is utilized the thermal aberrations in the phase correction device can render the phase correction device un-usable. The second driving issue is that the difficulties associated with coating the phase correction device are typically alleviated by making the high power beam path physically larger to reduce the power density on the phase correction device. This has many consequences and as the beam path grows in size the total size of the system grows tremendously, leading to very heavy systems whose size is un-necessarily larger to accommodate an adaptive optical system. A method that could avoid compensation in the high power beam path and perform all higher order correction functions in a low power beam path would have great benefit for a broad range of applications including long range laser radar, laser range-finding, and directed energy—these applications would become viable if high performance small size correction devices could be utilized.
On the topic of the second limitation—that the vast majority of methods are not effective with non-cooperative targets—there has been some recent headway made against this problem (see reference 9; 11), however the practical implementation of such methods remains challenging. In the typical/ideal scenario, a cooperative point source beacon projected from the target is used to make wavefront sensing measurements of the distortions along the path for pre-compensation of a laser beam by the adaptive optical system. However, many potential applications of adaptive optical systems, including laser radar, laser range finding, directed energy, and ophthalmic imaging all have “non-cooperative” targets. In the non-cooperative target case, no laser beacon is available from the target except that obtained from back-scattered radiation from the target itself or from the atmosphere (laser guide star obtained from Rayleigh or Mie light scattering). Many fundamental challenges exist in the case of a non-cooperative target. Overcoming these challenges would have significant benefit for many applications and open up the enabling capability for adaptive optical systems to new regimes and applications.
What is needed is a method for simultaneous compensation of aberrations in a high energy laser and for propagation through a turbulent medium with a cooperative or non-cooperative target. The present invention meets these needs by providing a method offering tremendous potential improvements both in terms of performance enhancements with non-cooperative targets and in reduction in size, weight, and power of a high energy laser system due to reductions in size, weight, and complexity of the beam control system.
U.S. patent application Ser. No. 12/234,041 filed Sep. 19, 2008 is fully incorporated herein by reference and provides a wavefront sensing and control technique to measure the aberrations along the propagation path using return from an ultra-short coherence length laser forming a controllable focused laser beacon at a non-cooperative target, regardless of the surface depth of the target. The present invention provides for an alternate but similar method for wavefront sensing and control using return from lasers with a short, but not necessarily ultra-short pulse, where the requirement for the pulse and corresponding coherence length is based on the target shape and orientation.